When should I apply for an extension of my NON-B visa in Thailand, and what documents do I need?

To apply for an extension of your NON-B visa based on employment, you can start the application process 30 days prior to the expiration date, which in your case is until 8th July. You should visit the immigration office within that 30-day window. Make sure to gather the necessary documentation, including paperwork from your employer, which may involve tax payments, social security obligation payments, and company-related documentation.